1. Field of the Invention
This inventicn relates to a device for controllably heating an oxygen sersor provided in an exhaust pipe of an internal combustion engine and heated by an electric heater to generate the output proportional to the oxygen concentration in exhaust gas.
b 2. Description of the Prior Art
Since oxygen concentration in exhaust gas of an internal combusticn engine is satisfactorily correlative to air fuel ratio in the region of the air fuel ratio larger than the theoretical one, i.e. lean mixture, the air fuel ratio in the exhaust gas can be detected accurately by measuring the oxygen concentration in the exhaust gas in such region. For an oxygen sensor measuring the oxygen concentration in the exhaust gas in such region is used a sensor comprising a bottomed tubular sensor element consisting a permeable measuring electrode provided at the exhaust gas-to-be-measured side, a permeable opposite electrode provided at the reference gas side, for example, the atmosphere side having known oxygen concentration and a solid electrolyte, for example, stabilized zirconia interposed between both electrodes. When current is supplied between both electrodes in such oxygen sensor, the oxygen can be moved through the electrolyte in one direction and the current can be maintained approximately at a specified value in a region of applied voltage by coating this permeable measuring electrode with a microporous diffusion resistance layer having oxygen sending capacity smaller than that of the permeable measuring electrode. This current value is referred to threshold current value which linearly varies approximately in proportion to oxygen concentration so that the oxygen concentration can be continuously detected from the change in the current value. On the other hand, in this oxygen sensor, it is necessary for generating current value proportional to the oxygen concentration in exhaust gas with a certain applied voltage to heat the sensor element to at least about 650.degree. C. and maintain it at the active condition.
For this end, an electric heater is provided in a tubular sensor element of the oxygen sensor and supplied with current from a storage battery as power source mounted on a vehicle to heat the sensor element. However, the voltage of the battery varies normally over the wide range from 10 to 16 V corresponding to the running condition of the engine and the charged condition of the battery. Thus, when power is supplied from the battery directly to the heater, the sensor element may be overheated, causing thermal deterioration. FIG. 1 shows the relationship between the applied voltage and element temperature of the heater. As mentioned above, the sensor element is inert at temperature lower than 650.degree. C., but may be subjected to thermal deterioration in at least 850.degree. C. Thus, it is necessary for maintaining the temperature of the sensor element within proper range of 650.degree.-800.degree. C. to maintain the applied voltage of the heater within the range of 10-14 V.
For this end, a temperature sensor such as thermocouple, thermistor or the like are mounted directly on the outer or inner surface of the sensor element of the oxygen sensor to detect the temperature of the sensor element, and current supplied from the battery to the heater is controllably turned on or off on the basis of the output signal of the temperature sensor so as to maintain the sensor element within the range of proper temperature. However, this heating control brings about a complicate construction of the oxygen sensor assembly in connection of the temperature sensor and lead wire, leading-out of lead wire, etc, presenting problems in reliability of the function.
Though a power supply circuit may be considered to apply a certain voltage to the heater for maintaining the sensor element of the oxygen sensor at a proper temperature, it has disadvantages in that the scale of the circuit is enlarged and cost is substantially increased.